Ion, halonium

In 1894, Hartmann and Meyer328 1 were the first to prepare a diphenyliodonium ion salt when they reacted iodosobenzene in concentrated H2SO4 and obtained para-iodophenylphenyliodonium bisulfate. Diphenyliodonium ions have since been stud- 

Open-chain (acyclic) dialkylhalonium ions of the type R2X+ (X = Cl, Br, I) were unknown until the 1960s, as were alkylarylhalonium ions (ArRX+). Realization of their possible role as intermediates in alkylation reactions of haloalkanes and -arenes has followed their preparation and study.333 

The bromonium ion concept was quickly used by other investigators to account for stereospecific transformation of alkenes, notably by Winstein and Lucas, but 

The halogen atom in organic halonium ions is generally bound to two carbon atoms, although in the case of acidic halonium ions—that is, protonated alkyl halides—one ligand is hydrogen. 

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Because the nature of the halogen atom can be varied, these salts show useful selectivity in their alkylation reaetions. We also prepared other halonium ions and studied their alkylating ability. 

Halonium ions, including hydrido or alkylhalonium ions, are similarly protolytically activated, indicative of protonation of the non-bonded electron pairs of their halogen atoms. 

MECHANISM OF HALOGEN ADDITION TO ALKENES HALONIUM IONS... 

Mechanism of Halogen Addition to Alkenes Halonium Ions... 

Halohydnn formation as depicted m Figure 6 13 is mechanistically related to halo gen addition to alkenes A halonium ion intermediate is formed which is attacked by water m aqueous solution... 

The regioselectivity of addition is established when water attacks one of the car bons of the halonium ion In the following example the structure of the product tells us that water attacks the more highly substituted carbon... 

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... 

Bromonium Ion (Section 6 16) A halonium ion in which the halogen is bromine (see halonium ion)... 

Halonium ion (Section 6 16) A species that incorporates apos itively charged halogen Bridged halonium 10ns are inter mediates in the addition of halogens to the double bond of an alkene... 

Cations of the type R R X+ are given names derived from the halonium ion, H2X+, by substitution, e.g., diethyliodonium chloride for (C2H5)2UCF. 

Bromine and chlorine convert the 1- and 2-butenes to compounds containing two atoms of halogens attached to adjacent carbons (vicinal dihahdes). Iodine fails to react. In this two-step addition mechanism the first step involves the formation of a cation. The halonium ion formed (a three-membered ring) requires antiaddition by the anion. 

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

Halogenated Butyl Rubber. Halogenation at the isoprene site ia butyl mbber proceeds by a halonium ion mechanism leading to a double-bond shift and formation of an exomethylene alkyl haUde. Both chlorinated and brominated mbber show the predominate stmcture (1) (>80%), by nmr, as described eadier (33,34). Halogenation of the unsaturation has no apparent effect on the isobutylene backbone chains. Cross-linked samples do not crystallize on extension due to the chain irregularities introduced by the halogenated isoprene units. 

Interpretation of reaction stereochemistry has focused attention on the role played by bridged halonium ions. 

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

Halohydrin fonnation, as depicted in Figure 6.13, is mechanistically related to halogen addition to alkenes. A halonium ion intennediate is fonned, which is attacked by water in aqueous solution. 

When treated with bromine or chlorine in aqueous solution, alkenes are converted to vicinal halohydrins. A halonium ion is an intermediate. The halogen adds to the carbon that has the greater number of hydrogens. Addition is anti. 

Anti addition is observed through a halonium ion intermediate. 

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

Initiator reactivity orders can be explained on the baas of differences in the rate of displacement of MeX from Et2AlX MeX complexes by f-BuX and/or the rate of ionization of Et2 A1X f-BuX complexes. PIB yields decrease with increase of Mel or MeBr concentration. This poisoning effect has been attributed to the formation of propagation-inactive halonium ions. 

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